1. Field of the Invention
The present invention relates to a permanent magnet stepping motor which is small in vibration as well as in rotational fluctuation and which is high in output torque, and particularly relates to a stepping motor which is suitable for direct driving.
2. Description of the Prior Art
Referring to FIGS. 1 and 2, the contents and configuration of the conventional technique of the stepping motor will be described.
Stepping motors have been widely used for inexpensive positioning, inexpensive velocity controlling, and inexpensive constant-speed synchronization controlling, because they can be used in an open loop.
Recently, as a low cost actuator for constant-speed synchronization control, a stepping motor is begun to be used, in place of a brushless DC motor, for driving a drum in a laser beam printer or the like.
Compared with a brushless DC motor, however, stepping motors are generally large in vibration as well as in noise, and how to reduce such vibration and noise has become a problem to be solved.
In the market, a two-phase hybrid stepping motor and a two-phase bobbin coil system permanent magnet stepping motor of the claw pole type and can stake type have been handled widely. However, the stepping motor of the former type has such a disadvantage that it is large in vibration as well as in noise while it is high in output, and the stepping motor of the later type has such a disadvantage that the air gap between its rotor and stator cannot be so narrowed because its stator is of the claw pole type so that its output torque is small while it is comparatively small in vibration as well as in noise.
As one of measures to eliminate the foregoing disadvantages, there has been proposed a three-phase hybrid stepping motor. FIGS. 1 and 2 show the structure of such a three-phase hybrid stepping motor.
FIG. 1 is a cross section showing the stepping motor cut in the direction perpendicular to a rotary shaft thereof, and FIG. 2 is a longitudinally sectional front view of the stepping motor showing its stator in section but showing its rotor not in section.
In FIGS. 1 and 2, a stator 1 is constituted by six poles 1-1 through 1-6 and coils 3 are wound on the respective poles to form a three-phase winding.
Each of rotor magnetic bodies 2-1 and 2-2 is constituted by a lamination of magnetic iron plates or the like and is provided at its outer periphery with a plurality of salient pole magnetic teeth. The magnetic bodies 2-1 and 2-2 are arranged in a manner so that the magnetic bodies 2-1 and 2-2 sandwich a disc-like permanent magnet 4 therebetween and the magnetic bodies 2-1 and 2-2 are rotationally shifted from each other by a 1/2 tooth pitch in position of their magnetic teeth.
The permanent magnet 4 is magnetized in the rotary shaft direction in such manner that, for example, the magnetic body 2-1 becomes S pole and the magnetic body 2-2 becomes N pole respectively. In comparison with a two-phase hybrid stepping motor, a three-phase hybrid stepping motor has an advantage that its vibration during rotation is small, and it is superior in cost performance particularly in the case of six-pole structure. An example of this structure is disclosed, for example, in U.S. Pat. No. 5,128,570.
Such a conventional three-phase hybrid stepping motor has a problem that it has a vibration band in rotation yet in a low speed range lower than 300 rpm while it is small in vibration in comparison with a two-phase hybrid stepping motor. Further, even in a high speed range not lower than 300 rpm, the conventional three-phase hybrid stepping motor is inferior in vibration and noise property to a brushless motor. Further, the torque of the conventional three-phase hybrid stepping motor becomes low remarkably in the high speed range not lower than 300 rpm.
The reason why such a disadvantage is generated in such a conventional three-phase hybrid stepping motor is that since its rotor is of the hybrid type, each of the magnetic bodies 2-1 and 2-2 of FIG. 2 is constituted by a lamination of magnetic iron plates to thereby partially form a magnetic path and inductance viewed from the winding becomes larger so that an exciting current hardly flows to lower the torque as the input frequency to the stator coil becomes higher.
Further, one of the reasons why vibration is large in such a conventional three-phase hybrid stepping motor in comparison with a brushless motor or the like is that since its rotor has a hybrid structure, each of the magnetic bodies 2-1 and 2-2 has magnetic teeth and the distribution of magnetic flux in the air gap between the stator and rotor has a square or trapezoidal waveform in accordance with the shape of the teeth so that higher harmonic components of the waveform are induced.